In an electrophotographic device or electrostatic recording device, an electrostatic latent image is formed on a photoconductor, to which toner is attracted. The toner is transferred to a support material, such as a piece of paper, and then fused to the support material by heat and thus a toner image is formed. There has been increasing demand to reduce energy consumption. As users' printing demands increase, printers are required to print at higher speeds, the toner must be heat/pressure fused to the paper in ever shortening times. A solution is to use toner with lower melting temperature to overcome this problem. However lower melting temperature toners suffer from a blocking problem. They tend to fuse together during storage at typical environmental conditions. Encapsulated or core-shell toners were developed to deal with that problem. The term “core/shell toner” refers to a toner powder comprising a core material, typically a wax, to act as a release agent, and a shell coating that includes a binder and the colorant for the toner particle.
Another benefit of encapsulated toners is to passivate the pigment charging, that is the charging characteristics of the toner particles is ultimately controlled by that of the colorants, especially those exposed at the surfaces of the toner particles. Influence of the pigment charging can be prevented by encapsulation of various color particles with a common shell polymer of the desired charging properties. Therefore, developer charging, including triboelectric and admix characteristics, can be controlled and pre-selected.
The preparation of encapsulated toners was first suggested in British Patent 1,319,815. The method of the British patent comprises preparing a kneaded oil phase component made up of one or more liquid resin monomers, coloring material, the polymerization initiator and a finely-divided inorganic dispersion stabilizer such as a metal powder or inorganic salt or oxide and a polar resinous additive which is soluble in a monomer. After suspension polymerization of the monomer, if required, the finely divided dispersion stabilizer is removed by dissolution in an acid and the polymer particles are removed from the aqueous phase and dried to produce toner.
It has also been proposed that a suspension polymerization process similar to the above referenced British Patent but not making use of an inorganic stabilizer be carried out to produce an encapsulated toner. This process is performed generally be mixing a monomer, a colorant and an initiator to form an oil soluble organic phase; dispersing this oil soluble phase in controlled size between 5 to 20 microns in a water phase, employing a suspending agent, for example polyvinyl alcohol; polymerizing, employing conventional suspension polymerization techniques; introducing a second monomer which is allowed to diffuse into the first polymer and consequently swells the polymer; introducing a water soluble initiator; and heating this reaction mixture to effect a polymerization of the second monomer and form the desired toner. It is found that the second initiator, the water soluble initiator, generates a free radical which attacks the surface of the swollen polymer particle and promotes polymerization at the surface by reacting with monomer at the surface thereby decreasing the monomer concentration and causing the transport of monomer to the surface by diffusion. The process is found to be self-terminating when the total amount of absorbed monomer has been converted to polymer at the surface, thus providing an encapsulated toner.
In U.S. Pat. No. 3,974,078 Crystal disclosed an encapsulated toner making process, by providing an electrostatographic toner of the colored resinous material in which at least a major portion of the resinous material is a heterogeneous (two phase) physical mixture comprised of at least one soft deformable polymer dispersed and encapsulated in a matrix of at least one tough polymer, as a plurality of discrete domains, instead of as a single core.
More particularly, the resinous portion of the toner includes two incompatible polymers one of which is employed as a matrix and the other of which is dispersed in a plurality of discrete domains and encapsulated in the matrix. The matrix component is a tough material to provide the physical properties which prevents blocking of and provides structural rigidity to the toner, and the dispersed component, is a soft material which provides the desired fixing properties. In the use of such a two-phase resin system, it has been found that the softer material must be completely encapsulated within the tough matrix material, and in addition, it has been found that the manner in which the softer component(s) is dispersed within the matrix influences the properties of the toner. The dispersion of the encapsulated softer component(s) as a plurality of domains of controlled size, results in an improved toner; in particular, a toner which is capable of being fixed by the application of pressure.
The toner of U.S. Pat. No. 3,974,078, in general, the tough polymer, which is employed as a matrix, has a glass transition temperature (Tg) of greater than about 50° C., preferably from about 55 to about 180° C., and more preferably from about 60 to about 90° C. If the polymer is crystalline, then the melting temperature (Tm) is greater than about 40° C., preferably from about 50 to about 90° C. and more preferably from about 60 to about 70° C. Thus, if the tough polymer is crystalline, the Tm must be greater than about 40° C. The molecular weight (number average) of the tough polymer block is greater than 1500, generally from about 5,000 to about 300,000, and preferably from about 10,000 to about 100,000.
In U.S. Pat. No. 4,148,741, Bayley disclosed a polymerization and attrition method for producing toner with reduced processing steps. In U.S. Pat. No. 4,148,741, Bayley identified that in formation of toner by known processes such as emulsion polymerization, spray drying or attrition from bulk, the problem of elements on the surface of the particle acting in a hydrophilic manner remains. Elements such as exposed pigment reactive groups, solvent or reactive monomers may attract water molecules and contribute to blocking of the toner and changes in triboelectric properties. Thus, U.S. Pat. No. 4,148,741 disclosed ways to produce encapsulated toners of high resistance to blocking, with minimized humidity sensitivity, low manufacturing cost and fewer steps.
Encapsulated, and cold pressure fixable toner compositions are known. Cold pressure fixable toners have a number of advantages as compared to toners that are fused by heat, primarily relating to the requirements for less energy since these toner compositions can be fused at room temperature. Nevertheless, many of the prior art cold pressure fixable toner compositions suffer from a number of deficiencies. For example, these toner compositions must usually be fused under high pressure, which has a tendency to severely disrupt the fusing characteristics of the toner selected. This can result in images of low resolution, or no images whatsoever. Also, with some of the prior art cold pressure toner compositions substantial image smearing can result from the high pressures required and from the use of plasticizer type materials in large quantities. Additionally, the cold pressure fixing toner compositions of the prior art have other disadvantages in that, for example, these compositions when used for development cause in some instances images with high gloss that are of low crease resistance. Furthermore, the images resulting exhibit an undesirable carbon paper effect, thus there is a total or partial image transfer from the image substrate to neighboring substrates caused by pressures arising from normal handling.
Wakamiya, et al. in U.S. Pat. No. 4,656,111 disclosed a pressure-fixable toner comprising a combination of a compound A having a hydrocarbon chain and a compound B having an amino group in addition to a hydrocarbon chain is excellent in dispersibility of colorant, pressure fixing characteristic and developing characteristic. Further, an encapsulated pressure-fixable toner comprising a core of the pressure-fixable toner particles and a shell encapsulating the core is also excellent in these properties.
A characteristic feature of the toner of U.S. Pat. No. 4,656,111 is that it has a particularly excellent dispersibility of a pigment, as compared with the toner of the prior art. This is because the compound B having a hydrocarbon chain and an amino group has an extremely good compatibility with a compound having a hydrocarbon chain and also has good affinity for a pigment. Thus, dispersion can be effected easily even in the combination of a pigment and a compound having a hydrocarbon chain in which the pigment can be poorly dispersed in the prior art. While the improvement may be attributable to improved dispersion of the pigment, the toner of U.S. Pat. No. 4,656,111 is excellent in mechanical strength, satisfactory in pressure fixing characteristic and sufficient in developing characteristic, with the change in developing characteristic being small even when used for a large number of times. The fixing characteristic is also excellent and does not change depending on the speed, paper quality, etc.
Breton, et al. in U.S. Pat. No. 4,766,051 described a cold pressure fixable colored toner composition comprised of a core containing a polymer in which is dispersed pigment particles selected from the group consisting of cyan, magenta, red, yellow pigments, and mixtures thereof, other than carbon blacks and magnetites; and encapsulated within a polymeric shell formulated by an interfacial polymerization.
The advantages of U.S. Pat. No. 4,766,051 include:
cold pressure fixable toner compositions with hard shells formulated by an interfacial polymerization process;                encapsulated toners with colored newsprint inks containing polymers, and economical pigments therein;        economical processes for the preparation of colored encapsulated toners by interfacial polymerization processes; and        
encapsulated toners with compatible color oil soluble dyes, which can be trapped within block copolymer domains, and wherein the block copolymer micro domains are “mini-reservoirs” of ink, like material.
Hsieh, et al. in U.S. Pat. No. 4,851,318 disclosed an improved process for the preparation of encapsulated toner compositions which comprises mixing core monomer(s), an initiator, or initiators, pigment particles, and oil soluble shell monomer(s); homogenizing the aforementioned mixture into an aqueous surfactant solution resulting in an oil-in-water suspension; thereafter adding water soluble shell monomer(s) to the oil-in-water suspension enabling an interfacial polymerization reaction between the oil soluble and the water soluble shell monomer(s); subsequently adding a low molecular weight polyethylene oxide surfactant protective colloid; and thereafter affecting a free-radical polymerization of the core monomer(s) by heating.
U.S. Pat. No. 4,851,318 teaches processes for encapsulated toner compositions comprised of a core containing a pigment particles, and a polymerizable monomer, or monomers, such as free-radical polymerizable monomer, or monomers; a polymerization initiator or initiators; and a shell generated by interfacial polymerization processes. More specifically, the process of U.S. Pat. No. 4,851,318 comprises (1) mixing a blend of a core monomer, or monomers, preferably not exceeding five, initiator, pigment particles, and oil soluble shell monomer(s); (2) forming an oil-in-water suspension by homogenization of the aforementioned mixture and an aqueous surfactant solution; (3) subsequently subjecting the aforementioned suspension to an interfacial polymerization by addition of a water soluble shell monomer, or monomers. After the polymerization is complete, (4) there is added to the suspension a low molecular weight polyethylene oxide surfactant as a protective colloid. Free-radical polymerization (5) is initiated by heating the suspension, for example, to 75° C., thus commencing the free-radical polymerization of the core monomer, or core monomers.
The benefits of encapsulated toners include the separation of functionality for core and shell. The shell enables mechanical robustness, anti blocking properties (thermal stability), uniform tribo charging for different colors (eliminates pigments, waxes, etc from the toner surface), good powder flow. The core enables lower fusing temperatures, release from fuser (wax in core), color properties.
However there are still some drawbacks with encapsulated toners that require solution. Incompatibility of the shell and core results in low gloss, low transparency; and non-uniform coverage of the shell.
Azizi et al in U.S. Published Patent Application 2001/0041260 disclosed a high clarity image bearing sheet, designed specifically to correct the drawback of encapsulated toners. Azizi et al provide a recording sheet including an additive, referred to herein as a compatibilizer, to improve the quality of images formed by toner powder development of electrostatic charge patterns. Recording sheets, carrying images produced by toner powder transfer and fusion on a receptor surface, according to the present invention, exhibit improved light transmission and reduced light scattering. Specifically, a transparent sheet is provided having a toner-receptive coating containing about 4 wt. % to about 25-wt % of a compatibilizer on at least one surface, wherein the coating has a low-density yellow Q factor value at least 2 less than an identical coating without the compatibilizer. Azizi et al is particularly effective in systems using core/shell toners where the core and the shell form an immiscible heterogeneous blend after fusing, with high levels of light scatter.
Toners used in a full-color copying apparatus are required to show a good color reproducibility and also sufficient color-mixing characteristic among the respective colors in a hot-pressure fixing step without impairing a clarity required for overhead projector (OHP) images. Compared with a black toner for ordinary monochromatic copying apparatus, a toner for full-color image formation preferably comprises a low-molecular weight binder resin having a sharp-melting characteristic. However, an ordinary sharp-melting binder resin shows only a low self-cohesion so that it is liable to cause a problem in anti-high-temperature offset characteristic when the toner is melted in a hot-pressure fixing step. In an ordinary black toner for monochromatic copying apparatus, a wax component having a relatively high crystallinity as represented by polyethylene wax or polypropylene wax is used as a release agent. However, in a toner for full-color image formation, because of a high crystallinity of the release agent per se or a difference in refractive index from an OHP sheet, the clarity of a projected image is impaired to result in projected images having low saturation and brightness.
Inaba, et al. in U.S. Pat. No. 5,635,325 disclosed a toner for developing electrostatic images includes: at least a binder resin, a colorant and an ester wax. The ester wax is contained in 3-40 wt. parts per 100 wt. parts of the binder resin. The ester wax includes ester compounds represented by a formula of R1—COO—R2, wherein R1 an R2 independently denote a hydrocarbon group of 15-45 carbon atoms. The ester wax contains 50-95 wt. % thereof of ester compounds having an identical number of total carbon atoms. The toner is especially characterized by low-temperature fixability, wide non-offset temperature range, good color mixing characteristic and transparency.
There is a need to provide encapsulated toners that provide all the advantages mentioned above while providing images with brilliant color and high clarity, without the need for specialized substrates.
There are specific needs to provide encapsulated toners without blocking problem, with low temperature fusing, and yet where the core and shell materials are fully compatibilized on the receiver sheets, without the need for specialized receivers.
The present invention provides a solution to the above mentioned problems.
It is an object of this invention to provide encapsulated toner compositions with many of the advantages illustrated herein.
It is also an object of this invention to provide toner compositions that can be made by virtually all the known fabrication methods.
In another object of this invention there are provided encapsulated toner compositions comprised of a core of resin binder, pigments/and or dyes surrounded by a polyester shell, whereas the core material incorporate a percentage of organic monomeric glasses sufficient to render the core and shell materials fully compatible once the electrophotographic printing process is completed.
A further object of the present invention is to provide heat fixable microcapsule toner compositions, which offer low flow characteristics.
An additional object of the present invention is to provide encapsulated color and black toner compositions that offer excellent image quality such as high fix, high resolution, high gloss, high color chroma, high transparency projection efficiency and other desirable color toner characteristics.
In another object of the present invention there are provided developer compositions formulated by admixing carrier particles with the toner compositions obtained by the processes illustrated herein.
Further, another object of the present invention is the provision of microcapsule toners with excellent surface release and powder flow properties without surface additives such as Aerosols.
Further embodiments of the present invention include a process for the preparation of encapsulated toner compositions which comprises a core component comprising an organic monomeric glass composition with tailored properties to maintain low melting, good transfer, with the additional benefits of compatibilizing the final printed toner material on the substrate to provide brilliant and transparent colors without scattering.